RCCB Logic

jd9

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I need some help to understand how my Consumer Unit should react to an interruption to the electricity supply to my house...
My Distribution Board has a 100A 100mA Time-delayed RCCB Incomer feeding 80A 30mA RCCB. When the electricity supply to the house fails, the 80A 30mA RCCB trips to OFF but the 100A 100mA (main) RCCB stays made (ON). Is this correct for a healthy system? The kitchen fridge/freezers - supplied by the 30mA side- are left with no power when the supply returns. Should the 80/30/RCCB stay 'latched' in the ON position when the 100/100/RCCB is manually tripped (not using the 'TEST' button)?
Kind regards, John
Configuration as follows...
100A 100mA Time-delayed RCCB Incomer feeding 80A 30mA RCCB
Time Delayed Split Load 18way Board
100A 100mA Time-delayed RCCB Incomer feeding...
C32 MCB (Air Con unit)
B32 MCB (Garage Power)
B6 (Lightning Upstairs)
B6 (Lighting Downstairs)
B6 (Kitchen Lighting)
80A 30mA RCCB feeding...
B20 (Conservatory Floor Heat)
B20 (Utility Sockets Radial)
B32 (Kitchen Sockets)
B32 (Downstairs Sockets)
B32 (Upstairs Sockets)
B32 (Ovens)
B32 (Hob)

[net]123648/12878_52267073.jpg View media item 12878[/net]
 
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It could just be a slight surge causing the RCD to trip - it is pretty common. As the 30mA device is more sensitive than the 100mA one then it will be more prone to tripping.
Your board is made by Controlgear Direct - they are not the best ones on the market and for some reason someone has stuck a Merlin Gerin MCB in there which means the unit isn't fully type tested.
 

jd9

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Cheers Spark123...thanks for the info on the manufacturer - I was assured it was 'good quality' by the installer.
If no circuits were connected at all other than the supply, would the 80A 30mA still trip when the power goes off (or the 100Amp 100mA breaker is manually tripped)?
 
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CGD are not the best consumer units in the world - they are budget (and that is being nice!)
I don't think the RCD will trip, it is more likely something plugged in causing a surge etc. The RCDs in consumer units are usually passive devices i.e. they will close without power applied unlike active ones as found in most of the plug in adaptors.
Try it - switch off the main RCD and then see if the 30mA one will close.
 
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As already said there are passive and active devices turn off all MCB's on the 30ma and then switch the main 100ma off and on does it trip then?

If you use RCBO's see here then the leakage in each device is limited to only that circuit so is less likely to trip. But it would require some work in changing the and not really a DIY job.
 

jd9

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Many thanks to spark123 and ericmark for their help. As suggested, I have carried out some more tests as follows:
1. With no power applied to Consumer Unit, 80A\30mA RCCB will stay in either ON (closed) or OFF (tripped) position. The TEST function (obviously) has no effect.
2. With all load MCBs OFF (tripped), 80A/30mA RCCB stays in the ON position when Incomer 100A\100mA RCCB is manually tripped to OFF and restored to ON.
3. With any MCB (connected to a loaded circuit) on either bus is ON, the 80A/30mA RCCB trips to OFF when power applied to CU by closing incomer 100A\100mA RCCB to ON.

I've learned that the 30mA RCCB should can and probably should stay ON when the power supply to the house is interrupted but still no idea why mine doesn't. Any more thoughts chaps?
Kind regards, John
 
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Since any MCB can cause fault I would suspect a fault on a neutral. The MCB's only switch the line wire so any neutral earth fault will always be present and with a TN-C-S supply at the consumer unit the difference between neutral and earth voltages are so small it may not trip the RCD. But with the inrush when turning on it may. To find the fault needs instruments which are not really DIY types and I think you will need an electrician to find fault.
 

jd9

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Thanks ericmark for your reply. Do you think selective use of RCBO on each circuit in turn would at least find which circuit has the neutral fault? Perhaps an electrician could disconnect each circuit in turn and repeat my tests to find the culprit?
At least I feel better informed to understand the problem when I find an electrician to fix the fault - even if TN-C-S earthing did stretch my brain a bit! Cheers.
 

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